Injection apparatus in use at present comprise a mechanism for closing and opening the die which is usually defined as a double toggle joint. In fact, in general they comprise four horizontal columns which are placed above a bed and which are positioned on the vertexes of a quadrilateral, being fixed by means of mechanical blocks which are called column nuts, the entire complex being mounted on a fixed plane usually called a lathe bed. A movable plate on which is fixed the moving part of the die, slides on the columns. This movable plate is connected by means of lever devices to the head of the die, which in its turn slides on the lathe bed, but with the possibility of being fixed to the columns due to the displacement of the column nuts, to the most convenient position on the basis of the dimensions of the objects to be manufactured by means of the apparatus.
In an apparatus as described hereinabove, the closing and opening movement of the movable plate which carries the die with respect to the fixed plate which carries the dolly is obtained by means of a complex of connecting rods and pins, usually called a double toggle joint, the motion being actuated by means of a hydraulic cylinder usually located on the die head. The action of this hydraulic cylinder, the piston of which is connected with the spider which slides on a column called a spider column, allows to obtain by means of the motion of the connecting rods, the extension and the regrouping and therefore the closure and the opening of the movable plate which carries the die. The number of gudgeon pins which connect the components of the connecting rods defines the structure, generally called toggle joints having five pivot points. It is clear that in a mechanism operating as described the course of the opening of the movable plate remains always conditioned particularly on the length of the connecting rod of rotation, because the mechanism must close on itself and the length cannot be greater than the limit of interference with the corresponding connecting rod of the opposite side. In view of the foregoing, any improvement in the extent of the course of the movable plate can only be obtained by increasing towards the exterior the length of the connecting rod of rotation and which connects the mechanism with a plate, the latter sliding on the lathe bed and being called the die head. These increases in the extent of the course therefore can only be obtained by increasing towards the exterior the length of the connecting rod of rotation, a fact which involves in general an increase in the dimension of the die head, with substantial increase in the cost of manufacture, in the occupied space, and above all creating flexure conditions which are frequently not acceptable because the applied forces are not aligned with the axis of the columns.